SKY87201_11_201856G.pdf

DATA SHEET
SKY87201-11: Low Noise, Fast Transient 600 mA Step-Down
Converter with Output Auto Discharge
Applications
Description
 Cellular phones
The SKY87201-11 is a 2 MHz step-down converter with an input
voltage range of 2.7 V to 5.5 V and output voltage as low as 0.6 V.
It is optimized to react quickly to a load variation. The SKY8720111 incorporates a unique low-noise architecture that reduces
ripple and spectral noise.
 Digital cameras
 Handheld instruments
 Microprocessor, DSP Core, I/O power
 PDAs and handheld computers
 USB devices
Features
 Output auto discharge when disabled
 VIN range: 2.7 V to 5.5 V
 Low-noise, light-load mode
 Low ripple power management mode
 Output voltage adjustable from 0.6 V to VIN
 No load quiescent current: 37 μA
 Up to 98% efficiency
 Maximum output current: 600 mA
 Switching Frequency: 2 MHz
 Soft start: 150 μs
The SKY87201-11 is programmable with external feedback
resistors. It can deliver 600 mA of load current while maintaining
a low 37 μA no-load quiescent current. The 2 MHz switching
frequency minimizes the size of external components while
keeping switching losses low.
The SKY87201-11 is designed to maintain high efficiency
throughout the operating range, which is critical for portable
applications.
The device has an output auto discharge feature. This enables the
device to quickly discharge the output when the device is
disabled (VEN = VIN to 0 V).
The SKY87201-11 is provided in a small, 8-pin, 2 x 2 mm Thin
Dual Flat No-Lead (STDFN) package.
A typical application circuit diagram is provided in Figure 1. The
pin configuration and package are shown in Figure 2. Signal pin
assignment and functional pin descriptions are provided in
Table 1. A functional block diagram is shown in Figure 3.
 Fast load transient
 Over-temperature protection
 Current limit protection
 100% duty cycle low-dropout operation
 Shutdown current: <1 μA
 Available in an STDFN (8-pin, 2 x 2 mm) package
(MSL1, 260 C per JEDEC J-STD-020)
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
L1
4.7 μH
Input Supply
Voltage
2.7 V ~ 5.5 V
C1
4.7 μF
VIN
LX
EN
OUT
AGND
R1
118 kΩ
Output
Voltage
1.8 V
C2
10 μF
R2
59 kΩ
PGND
Figure 1. Typical Application Circuit Schematic
N/C
1
8
EN
N/C
AGND
PGND
2
7
3
6
OUT
VIN
4
5
LX
Figure 2. SKY87201-11 Pinout – 8-Pin STDFN Package
(Top View)
Table 1. SKY87201-11 Signal Descriptions
Name
Pin #
N/C
1
Description
Not connected.
Name
Pin #
Description
LX
5
Switching node. Connect an inductor to this pin, which is
internally connected to the drain of both high- and lowside MOSFETs.
N/C
2
Not connected.
VIN
6
Input supply voltage for the converter.
AGND
3
Non-power signal ground pin.
OUT
7
Feedback input pin. This pin is connected to an external
resistive divider.
PGND
4
Main power ground return pin. Connect to the output and
input capacitor return.
EN
8
Enable pin
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
VIN
OUT
Err
Amp
.
DH
Voltage
Reference
LX
Logic
DL
EN
INPUT
PGND
AGND
S2945
Figure 3. SKY87201-11 Block Diagram
Electrical and Mechanical Specifications
The absolute maximum ratings of the SKY87201-11 are provided
in Table 2. Electrical specifications are provided in Table 3.
Typical performance characteristics of the SKY87201-11 are
illustrated in Figures 4 to 25.
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Table 2. Absolute Maximum Ratings (Note 1)
Parameter
Symbol
Minimum
Maximum
Units
+6.0
V
Supply voltage GND
VIN
LX to GND
VLX
–0.3
VIN + 0.3
V
OUT to GND
VOUT
–0.3
VIN + 0.3
V
EN to GND
VEN
–0.3
VIN + 0.3
V
Junction temperature
TJ
–40
+150
C
Soldering temperature (@ leads, 10 sec)
TLEAD
+300
C
Thermal resistance (Note 2)
ΘJA
70
C/W
Power dissipation (Note 2) (Note 3)
PD
1.43
W
Electrostatic Discharge:
ESD
1500
4000
400
V
V
V
Charged Device Model (CDM), Class 4
Human Body Model (HBM), Class 3A
Machine Model (MM), Class C
Note 1: Exposure to maximum rating conditions for extended periods may reduce device reliability. There is no damage to device with only one parameter set at the limit and all other
parameters set at or below their nominal value. Exceeding any of the limits listed here may result in permanent damage to the device.
Note 2: Mounted on an FR4 board.
Note 3: Derate 14.3 mW/C above 25 C.
CAUTION: Although this device is designed to be as robust as possible, Electrostatic Discharge (ESD) can damage this device. This device
must be protected at all times from ESD. Static charges may easily produce potentials of several kilovolts on the human body
or equipment, which can discharge without detection. Industry-standard ESD precautions should be used at all times.
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Table 3. SKY87201-11 Electrical Characteristics (Note 1)
(TA = –40 C to +85 C [Typical Values @ +25 C, VIN = 3.6 V], Unless Otherwise Noted)
Parameter
Symbol
Test Conditions
Minimum
Typical
Maximum
Units
5.5
V
2.7
V
Step-Down Converter
Input voltage
VIN
Under Voltage Lockout threshold
VUVLO
2.7
VIN rising
100
Hysteresis
VIN falling
1.8
IOUT = 0 mA to 600 mA,
VIN = 2.7 V to 5.5 V
–3
Output voltage tolerance
VOUT
Output voltage range
VOUT
Quiescent current
ICQ
No load
Shutdown current
ISHDN
EN = AGND = PGND
P-channel current limit
ILIM
mV
V
+3
0.6
VIN
37
%
V
70
μA
1
μA
800
mA
High side switch on resistance
RDS(ON)H
0.35
Ω
Low side switch on resistance
RDS(ON)L
0.30
Ω
Line regulation
ΔVLINEREG
VIN = 2.7 V to 5.5 V,
IOUT = 600 mA
0.1
%/V
Output threshold voltage accuracy
VOUT
0.6 V output, No load,
TA = 25 C
Output leakage current
IOUT
0.6 V output
Output impedance
ROUT
>0.6 V output
Start-up time
TS
From enable to output
regulation
Oscillator frequency
fOSC
TA = 25 C
Over-temperature shutdown threshold
TSD
140
C
Over-temperature shutdown hysteresis
THYS
15
C
591
600
609
mV
0.2
μA
kΩ
250
μs
150
0.9
2.0
2.6
MHz
Enable
Enable threshold low
VEN(L)
Enable threshold high
VEN(H)
0.6
1.4
Input low current
IEN
VIN = VOUT = 5.5 V
Output discharge time
TDISCHARGE
VIN = 3.0 V to 5.5 V,
VEN = VIN to 0 V,
VOUT = 3.0 V to 0.4 V,
L = 4.7 μH
COUT = 22 μF
–1
V
V
+1
μA
25
ms
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Typical Performance Characteristics
(TA = +25 °C, Unless Otherwise Noted)
1
100
0.8
80
70
60
50
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
40
30
0.1
Output Error (%)
Efficiency (%)
90
0.6
0.4
0.2
0
-0.2
-0.4
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
-0.6
-0.8
-1
1
10
100
1000
0.1
1
Output Current (mA)
100
1
90
0.8
80
70
60
VIN = 3V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
40
30
0.1
1
10
100
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0.1
1000
1
0.8
80
70
60
VIN = 3.6V
VIN = 4.2V
VIN = 5V
10
100
Output Current (mA)
Figure 8. Efficiency vs Output Current
(VOUT = 3.3 V)
1000
Output Error (%)
Efficiency (%)
1
90
1
100
VIN = 3.6V
VIN = 4.2V
VIN = 5V
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0.1
1
10
100
Output Current (mA)
Figure 9. Load Regulation
(VOUT = 3.3 V)
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1000
Figure 7. Load Regulation
(VOUT = 2.5 V)
100
30
0.1
10
Output Current (mA)
Figure 6. Efficiency vs Output Current
(VOUT = 2.5 V)
40
1000
VIN = 3V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
0.6
Output Current (mA)
50
100
Figure 5. Load Regulation
(VOUT = 1.8 V)
Output Error (%)
Efficiency (%)
Figure 4. Efficiency vs Output Current
(VOUT = 1.8 V)
50
10
Output Current (mA)
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1000
DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
1mA
400mA
600mA
0.4
0.3
Accuracy (%)
Switching Frequency (MHz)
0.5
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
2.08
2.06
2.04
2.02
2
1.98
1.96
1.94
1.92
-40
-15
10
Input Voltage (V)
Figure 10. Line Regulation
(VOUT = 1.8 V)
85
2.0
Output Voltage Error (%)
Frequency Variation (%)
60
Figure 11. Switching Frequency vs Temperature
(VOUT = 1.8 V, IOUT = 1 A)
4
3
2
1
0
-1
-2
VOUT = 1.8V
VOUT = 3V
-3
-4
2.7
3.1
3.5
3.9
4.3
4.7
5.1
1.0
0.0
-1.0
-2.0
-40
5.5
-20
0
Input Voltage (V)
40
60
80
100
Figure 13. Output Voltage Error vs Temperature
(VIN= 3.6 V, VOUT = 1.8 V, IOUT = 400 mA)
60
550
55
120°C
100°C
85°C
25°C
500
50
RDS(ON) (mΩ
Ω)
45
40
35
30
25
85°C
25C
-40°C
20
15
10
2.7
20
Temperature (°C)
Figure 12. Frequency Variation vs Input Voltage
Supply Current (µA)
35
Input Voltage (V)
3.1
3.5
3.9
4.3
4.7
5.1
450
400
350
300
250
5.5
Input Voltage (V)
Figure 14. No Load Quiescent Current vs Input Voltage
(VOUT = 1.8 V)
200
2.5
3
3.5
4
4.5
5
5.5
6
Input Voltage (V)
Figure 15. P-Channel RDS(ON) vs Input Voltage
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
1.9
RDS(ON) (mΩ
Ω)
450
Output Voltage (top) (V)
120°C
100°C
85°C
25°C
500
400
350
300
250
200
2.5
1.8
1.7
300mA
1.6
1mA
1.5
1.4
1.3
300mA
1.2
1mA
1.1
3
3.5
4
4.5
5
5.5
6
Output and Inductor Current
(100mA/div)
550
Time (50µs/div)
Input Voltage (V)
Output Voltage (top) (V)
1.75
1.7
400mA
300mA
1.65
1.6
400mA
1.55
300mA
1.5
1.85
1.8
1.75
1.7
300mA
Time (50µs/div)
Figure 19. Load Transient
(VIN= 3.6 V, VOUT = 1.8 V, COUT = 10 μF, CFF = 0 pF)
1.7
300mA
1.65
300mA
400mA
1.5
Time (50µs/div)
Figure 20. Load Transient
(VIN= 3.6 V, VOUT = 1.8 V, COUT = 10 μF, CFF = 100 pF)
Input Voltage (top) (V)
Output Voltage (top) (V)
1.75
4.8
1.92
4.2
1.9
3.6
1.88
3
1.86
2.4
1.84
1.8
1.82
1.2
1.8
0.6
1.78
0
1.76
Time (50µs/div)
Figure 21. Line Transient
(VOUT= 1.8 V, VIN = 3.6 V to 4.2 V, IOUT = 400 mA, CFF = 0 pF)
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Output Voltage (bottom) (V)
1.8
Output and Inductor Current
(100mA/div)
1.85
1.55
400mA
1.5
Figure 18. Load Transient
(VIN= 3.6 V, VOUT = 1.8 V, COUT = 4.7 μF, CFF = 0 pF)
1.6
300mA
1.6
1.55
Time (50µs/div)
400mA
400mA
1.65
Output and Inductor Current
(100mA/div)
1.8
Output and Inductor Current
(100mA/div)
1.85
Figure 17. Load Transient
(VIN= 3.6 V, VOUT = 1.8 V, COUT = 10 μF, CFF = 100 pF)
Output Voltage (top) (V)
Figure 16. N-Channel RDS(ON) vs Input Voltage
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0.35
0.01
0.3
-0.01
0.2
-0.02
0.15
-0.03
0.1
-0.04
0.05
-0.05
0
-0.06
-0.05
1
-0.01
0.8
-0.02
0.6
-0.03
0.4
-0.04
0.2
-0.05
0
-0.2
Time (200ns/div)
4
3
2
1
0
0.5
0
-0.5
Figure 23. Output Ripple
(VOUT= 1.8 V, VIN = 3.6 V, IOUT = 400 mA, CFF = 0 pF)
Input Current (bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
1.2
-0.06
Figure 22. Output Ripple
(VOUT= 1.8 V, VIN = 3.6 V, IOUT = 1 mA, CFF = 0 pF)
Figure 24. Soft Start
(VIN= 3.6 V, VOUT = 1.8 V, IOUT = 400 mA)
1.4
0.01
0
Time (10µs/div)
Time (100µs/div)
0.02
Inductor Current
(bottom) (A)
0.25
0
Output Voltage (top) (V)
0.02
Inductor Current
(bottom) (A)
Output Voltage (top) (V)
DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
EN
2V/div
VOUT
2V/div
4ms/div
Figure 25. Discharge Time
(CIN= 2.2 μF, COUT = 2.2 μF, L = 4.7 μH, R1 = 236 kΩ,
R2 = 59 kΩ)
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Functional Description
The SKY87201-11 is a high performance, 600 mA, 2 MHz
monolithic step-down converter. It has been designed to minimize
external component size and optimize efficiency over the
complete load range with reduced ripple and spectral noise. Apart
from the small bypass input capacitor, only a small L-C filter is
required at the output. Typically, a 4.7 μH inductor is
recommended for a 1.8 V output converter (see Table 4).
The SKY87201-11 can be programmed with external feedback to
any voltage, ranging from 0.6 V to the input voltage (VIN). An
additional feed-forward capacitor can also be added to the
external feedback loop to provide improved transient response.
At dropout, the converter duty cycle increases to 100 percent and
the output voltage tracks the input voltage minus the RDS(ON) drop
of the P-channel high-side MOSFET.
The input voltage range is 2.7 V to 5.5 V. The converter efficiency
has been optimized for all load conditions, ranging from no load to
600 mA.
constant output voltage for all load and line conditions. Internal
loop compensation terminates the transconductance voltage error
amplifier output.
Soft Start/Enable
Soft start limits the current surge seen at the input and eliminates
output voltage overshoot. When pulled low, the enable input
forces the SKY87201-11 into a low-power, non-switching state.
The total input current during shutdown is less than 1 μA.
Current Limit and Over-Temperature Protection
For overload conditions, the peak input current is limited. To
minimize power dissipation and stresses under current limit and
short-circuit conditions, switching is terminated after entering
current limit for a series of pulses. Switching is terminated for
seven consecutive clock cycles after a current limit has been
sensed for a series of four consecutive clock cycles.
The internal error amplifier and compensation provides excellent
transient response, load, and line regulation. Soft start eliminates
any output voltage overshoot when the enable or the input voltage
is applied.
Thermal protection completely disables switching when internal
dissipation becomes excessive. The junction over-temperature
threshold is 140 °C with 15 °C of hysteresis. Once an overtemperature or over-current fault condition is removed, the output
voltage automatically recovers.
Control Loop
Under-Voltage Lockout
The SKY87201-11 is a peak current mode step-down converter.
The current through the P-channel MOSFET (high side) is sensed
for current loop control, as well as short circuit and overload
protection. A fixed slope compensation signal is added to the
sensed current to maintain stability for duty cycles greater than
50 percent. The peak current mode loop appears as a voltageprogrammed current source in parallel with the output capacitor.
Internal bias of all circuits is controlled using the the VIN input.
Under-Voltage Lockout (UVLO) guarantees sufficient VIN bias and
proper operation of all internal circuitry before activation.
The output of the voltage error amplifier programs the current
mode loop for the necessary peak switch current to force a
Output Auto Discharge
When the SKY87201-11 is disabled (VEN =VIN to 0 V), the output is
quickly discharged (refer to Figure 26).
LX
150Ω
EN
PGND
Figure 26. Output Auto Discharge Circuit
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Evaluation Board Layout
3. The feedback trace or OUT pin should be separate from any
power trace and connected as close as possible to the load
point. Sensing along a high-current load trace degrades DC
load regulation. If external feedback resistors are used, they
should be placed as close as possible to the OUT pin to
minimize the length of the high impedance feedback trace.
The SKY87201-11 Evaluation Board is used to test the
performance of the SKY87201-11 Step-Down Converter. An
Evaluation Board schematic diagram is provided in Figure 27.
Component values for the SKY87201-11 Evaluation Board are
listed in Table 4. Typical surface mount inductors and capacitors
are listed in Tables 5 and 6, respectively. Table 7 provides the Bill
of Materials (BOM) for Evaluation Board components.
4. The resistance of the trace from the load return to PGND
should be kept to a minimum. This helps to minimize any
error in DC regulation due to differences in the potential of the
internal signal ground and the power ground. A high density,
small footprint layout can be achieved using an inexpensive,
miniature, non-shielded, high DCR inductor.
The suggested PCB layout for the SKY87201-11 is shown in
Figures 28 and 29. The following guidelines should be used to
help ensure a proper layout.
1. The input capacitor (C1) should connect as close as possible
to VIN and PGND.
2. C2 and L1 should be connected as close as possible. The
connection of L1 to the LX pin should be as short as possible.
Input
Voltage
6
8
×
C1
4.7 μF
1
4
VIN
LX
EN
OUT
N/C
N/C
AGND
PGND
Output
Voltage
5
L1
4.7 μH
7
2
R1
118 kΩ
×
C3
DNI
C2
4.7 μF
3
R2
59 kΩ
GND
GND
VIN
JP1
1
2
3
EN
GND
GND
Figure 27. SKY87201-11 Evaluation Board Schematic
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Table 4. Evaluation Board Component Values
VOUT (V)
R2 = 59 kΩ
R1 (kΩ)
R2 =221 kΩ
R1 (kΩ)
L1
(μH)
0.80
19.6
75.0
2.2
0.90
29.4
113
2.2
1.00
39.2
150
2.2
1.10
49.9
187
2.2
1.20
59.0
221
2.2
1.30
68.1
261
2.2
1.40
78.7
301
4.7
1.50
88.7
332
4.7
1.80
118.0
442
4.7
1.85
124.0
464
4.7
2.00
137.0
523
6.8
2.50
187.0
715
6.8
3.30
267.0
1000
6.8
Table 5. Typical Surface Mount Inductors
Max DC
Current
(A)
DCR
(Ω)
CDRH3D16-2R2
2.2
1.20
0.072
3.8x3.8x1.8
Shielded
CDRH3D16-4R7
4.7
0.90
0.105
3.8x3.8x1.8
Shielded
Part #
Sumida
Coiltronics
Type
Inductance
(μH)
Manufacturer
Size,
lxwxh
(mm)
CDRH3D16-6R8
6.8
0.73
0.170
3.8x3.8x1.8
Shielded
SD3118-4R7
4.7
0.98
0.122
3.1x3.1x1.85
Shielded
SD3118-6R8
6.8
0.82
0.175
3.1x3.1x1.85
Shielded
VLS3015T-4R7MR99
4.7
0.99
0.136
3.0x3.0x1.5
Shielded
VLS3015T-6R8MR86
6.8
0.86
0.176
3.0x3.0x1.5
Shielded
Wurth
744042006
6.8
1.25
0.100
4.8x4.8x1.8
Shielded
Taiyo Yuden
MAKK2016T4R7M
4.7
1.20
0.308
2.0x1.6x1.0
Shielded
TDK
Table 6. Surface Mount Capacitors
Manufacturer
Murata
Part #
Value
(μF)
Voltage
(V)
Temp. Coefficient
Case
GRM219R61A475KE19
4.7
10
X5R
0805
GRM21BR60J106KE19
10
6.3
X5R
0805
GRM188R60J475KE19
4.7
6.3
X5R
0603
GRM219R61A106KE44
10
10
X5R
0805
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Table 7. Evaluation Board Bill of Materials (STDFN Package)
Component
C1, C2
Part Number
GRM188R60J475KE19
Manufacturer
Murata
Description
Ceramic capacitors, 4.7 μF, 0603, X5R, 6.3 V, 10%
L1
CDRH3D16-4R7
Sumida
4.7 μH inductor, 105 mΩ, 0.9 A, 20%
R1
RC0603FR-0759KL
Yageo
59 kΩ resistor, 1/10 W, 1%, 0603, SMD
R2
RC0603FR-07118KL
Yageo
118 kΩ resistor, 1/10 W, 1%, 0603, SMD
Figure 28. SKY87201-11 Evaluation Board Component Side
Layout
Figure 29. SKY87201-11 Evaluation Board Solder Side Layout
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Application Information
C IN 
Inductor Selection
The step-down converter uses peak current mode control with
slope compensation to maintain stability for duty cycles greater
than 50 percent. The output inductor value must be selected so
the inductor current down-slope meets the internal slope
compensation requirements. The internal slope compensation of
the SKY87201-11 is 0.24 A/μs (see Equation 1). This equates to a
slope compensation that is 75 percent of the inductor current
down-slope for a 1.5 V output and 4.7 μH inductor.
0.75  Vo 0.75  1.5 V
m

 0.24 A / s
L
4.7 H
0.75  Vo
0.75  Vo

 3 s / A  Vo
m
0.24 A / s
(2)
Manufacturer specifications list both the inductor DC current
rating, which is a thermal limitation, and the peak current rating,
which is determined by the saturation characteristics. The
inductor should not show any appreciable saturation under normal
load conditions. Some inductors may meet the peak and average
current ratings yet result in excessive losses due to a high Direct
Current Resistance (DCR). Always consider the losses associated
with DCR and the affect on the total converter efficiency when
selecting an inductor.
The 4.7 μH CDRH3D16 series inductor selected from Sumida has
a 105 mW DCR and a 900 mA DC current rating. At full load, the
inductor DC loss is 17 mW, which gives a 2.8 percent loss in
efficiency for a 400 mA, 1.5 V output.
Input Capacitor
Select a 2.2 μF to 10 μF X7R or X5R ceramic capacitor for the
input. To estimate the required input capacitor size, determine the
acceptable input ripple level (VPP) and solve for CIN (see
Equations 3, 4, and 5). The calculated value varies with input
voltage and is a maximum when VIN is double the output voltage.
 1
  for V IN  2  VO
 4

VO 
V
 1  O
V IN 
V IN




For VIN = 2 × VO:
(3)
(4)
IO
2
The term VO/VIN x (1 – VO/VIN) appears in both the input voltage
ripple and input capacitor RMS current equations and is a
maximum when VO is twice VIN. This is why the input voltage
ripple and the input capacitor RMS current ripple are a maximum
at a 50 percent duty cycle.
The input capacitor provides a low impedance loop for the edges
of pulsed current drawn by the SKY87201-11. Low ESR/ESL X7R
and X5R ceramic capacitors are ideal for this function. To
minimize stray inductance, the capacitor should be placed as
closely as possible to the device. This keeps the high frequency
content of the input current localized, minimizing EMI and input
voltage ripple.
The proper placement of the input capacitor (C1) can be seen in
the Evaluation Board layout in Figure 28.
A laboratory test setup typically consists of two long wires running
from the bench power supply to the Evaluation Board input
voltage pins. The inductance of these wires, along with the lowESR ceramic input capacitor, can create a high Q network that
may affect converter performance. This problem often becomes
apparent in the form of excessive ringing in the output voltage
during load transients. Errors in the loop phase and gain
measurements can also result.
Since the inductance of a short PCB trace feeding the input
voltage is significantly lower than the power leads from the bench
power supply, most applications do not exhibit this problem.
In applications where the input power source lead inductance
cannot be reduced to a level that does not affect the converter
performance, a high ESR tantalum or aluminum electrolytic
should be placed in parallel with the low ESR/ESL bypass
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(6)

1
  D  1  D   0.5 2 

2

I RMS ( MAX ) 
Table 4 displays inductor values for the SKY87201-11.
Vo 
V
 1  o
V IN 
VIN
I RMS  I O 
VO 
V
 1  O
V IN 
V IN
In this case, a standard 6.8 μH values is selected.
C IN
The maximum input capacitor RMS current is calculated using
Equation 6:
The input capacitor RMS ripple current varies with the input and
output voltage and is always less than or equal to half of the total
DC load current as follows:
 3 s / A  2.5 V  7.5 H
Vo 
V 
  1  O 
V IN 
V IN 

 V PP



 I  ESR   FS
 O

(5)
Always examine the ceramic capacitor DC voltage coefficient
characteristics when selecting the proper value. For example, the
capacitance of a 10 μF, 6.3 V, X5R ceramic capacitor with 5.0 V
DC applied is actually about 6 μF.
(1)
This is the internal slope compensation for the SKY87201-11.
When externally programming the output voltage to 2.5 V, the
calculated inductance is 7.5 μH (from Equation 2).
L
1
 V PP



 I  ESR   4  FS
 O

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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
ceramic. This dampens the high Q network and stabilizes the
system.
Output Capacitor
The output capacitor limits the output ripple and provides holdup
during large load transitions. A 4.7 μF to 10 μF X5R or X7R
ceramic capacitor typically provides sufficient bulk capacitance to
stabilize the output during large load transitions and has the ESR
and ESL characteristics necessary for low output ripple.
The output voltage droop due to a load transient is dominated by
the capacitance of the ceramic output capacitor. During a step
increase in load current, the ceramic output capacitor alone
supplies the load current until the loop responds. Within two or
three switching cycles, the loop responds and the inductor current
increases to match the load current demand. The relationship of
the output voltage droop during the three switching cycles to the
output capacitance can be estimated by Equation 7:
COUT 
3  I LOAD
V DROOP  FS
(7)
Once the average inductor current increases to the DC load level,
the output voltage recovers. Equation 7 establishes a limit on the
minimum value for the output capacitor with respect to load
transients.
The internal voltage loop compensation also limits the minimum
output capacitor value to 4.7 μF. This is due to its effect on the
loop crossover frequency (bandwidth), phase margin, and gain
margin. Increased output capacitance reduces the crossover
frequency with greater phase margin.
The maximum output capacitor RMS ripple current is given by
Equation 8:
I RMS ( MAX ) 
1
2 3


VOUT  VIN ( MAX )  VOUT

L  F  VIN ( MAX )
(8)
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than a few
degrees rise in hot-spot temperature.
Adjustable Output Resistor Selection
For applications that require an adjustable output voltage, the
SKY87201-11 can be externally programmed. Resistors R1 and
R2 shown in the schematic diagram (see Figure 27) program the
output to regulate at a voltage higher than 0.6 V.
To limit the bias current required for the external feedback
resistor string while maintaining good noise immunity, the
minimum suggested value for R2 is 59 kΩ (see Equation 9).
Although a larger value will further reduce quiescent current, it
will also increase the impedance of the feedback node, making it
more sensitive to external noise and interference. Table 8
summarizes the resistor values for various output voltages with
R2 set to either 59 kΩ for good noise immunity or 221 kΩ for
reduced, no-load input current.
V

 1.5 V

R1   OUT  1  R 2  
 1   59 k  88.5 k
V
0
.
6
V


 REF

(9)
The SKY87201-11, combined with an external feed-forward
capacitor (C3 in Figure 27), delivers enhanced transient response
for extreme pulsed load applications. The addition of the feedforward capacitor typically requires a larger C2 output capacitor
for stability.
Thermal Calculations
There are three types of losses associated with the SKY87201-11
step-down converter: switching losses, conduction losses, and
quiescent current losses.
Conduction losses are associated with the RDS(ON)
characteristics of the power output switching devices. Switching
losses are dominated by the gate charge of the power output
switching devices. At full load, assuming Continuous Conduction
Mode (CCM), a simplified form of the losses is given by Equation
10:
PTOTAL 
I O 2  ( RDSON ( HS )  VO  RDSON ( LS )  [ VIN  VO ])
VIN
(10)
 ( t SW  F  I O  I Q )  VIN
Where IQ is the step-down converter quiescent current. The term
tSW is used to estimate the full load step-down converter
switching losses.
For the condition where the step-down converter is in dropout at
100 percent duty cycle, the total device dissipation reduces to:
PTOTAL  I O 2  RDSON ( HS )  I Q  VIN
Since RDS(ON), the quiescent current, and switching losses all vary
with input voltage, the total losses should be investigated over the
complete input voltage range.
Given the total losses, the maximum junction temperature can be
derived from the thermal resistance (ΘJA), which is 70 °C/W as
shown by Equation 11:
TJ ( MAX )  PTOTAL   JA  TA
(11)
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15
DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Table 8. Resistor Selection for Different Output Voltage Settings
VOUT
(V)
R2 = 59 kΩ
R2 = 221 kΩ
R1 (kΩ)
R1
0.80
19.6
75K
0.90
29.4
113K
1.00
39.2
150K
1.10
49.9
187K
1.20
59.0
221K
1.30
68.1
261K
1.40
78.7
301K
1.50
88.7
332K
1.80
118.0
442K
1.85
124.0
464K
2.00
137.0
523K
2.50
187.0
715K
3.30
267.0
1.00M
Step-Down Converter Design Example
I RMS 
Specifications
VO = 1.8 V @ 400 mA (adjustable using 0.6 V version), pulsed
load ΔILOAD = 300 mA
VIN = 2.7 V to 4.2 V (3.6 V nominal)
1
2 3
1
2 3



VOUT  V IN ( MAX )  VOUT
L1  F  V IN ( MAX )
1.8 V  4.2 V  1.8 V 
 32 mArms
4.7 H  2.0 MHz  4.2 V
Pesr  esr  I RMS 2  5 m  32 mA2  6 W
FS = 2 MHz
TA = 85 °C
Input Capacitor
Input ripple VPP = 25 mV
1.8 V Output Inductor
L1 = 3 μs/A x VO2 = 3 μs/A x 1.8 V = 5.4 μH (use 4.7 μH, see
Table 4)
C IN 
1
1

 2.17 F
 V PP

 25 mV


  4  FS

  4  2.0 MHz

5
m


ESR
 0.4 A

 I



 O

For Sumida inductor CDRH3D16, 4.7 μH, DCR = 105 mΩ
I L1

V
VO

 1  O
VIN
L1  FS 
I PKL1  I O 


1.8 V
1.8 V


 4.7 H  2.0 MHz   1  4.2 V


I L1
2

  109 mA

 0.4 A  0.055 A  0.455 A
PL1  I O 2  DCR  0.4 A 2  105 m  17 mW
VDROOP = 0.1 V
3  I LOAD
3  0.3 A


 4.5 F
V DROOP  FS 0.1V  2 MHz
(use 2.2 μF)
I RMS 
IO
 0.2 Arms
2
Pesr  esr  I RMS 2  5 m  0.2 A2  0.2 mW
SKY87201-11 Losses
PTOTAL 
1.8 V Output Capacitor
COUT

I O 2  ( RDSON ( HS )  VO  RDSON ( LS )  [ VIN  VO ])
VIN
 ( t sw  F  I O  I Q )  VIN
(use 4.7 μF)

0.4 2  ( 0.725   1.8 V  0.7   [ 4.2 V  1.8 V ]
4.2 V
 ( 5 ns  2.0 MHz  0.4 A  70 A )  4.2 V  131 mW


T J ( MAX )  T A   JA  PLOSS  85 o C  70 o C / W  131 mW  94.2 o C
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Package and Handling Information
Instructions on the shipping container label regarding exposure to
moisture after the container seal is broken must be followed.
Otherwise, problems related to moisture absorption may occur
when the part is subjected to high temperature during solder
assembly.
The SKY87201-11 is rated to Moisture Sensitivity Level 1 (MSL1)
at 260 C. It can be used for lead or lead-free soldering. For
additional information, refer to the Skyworks Application Note
Solder Reflow Information, document number 200164.
Production quantities of this product are shipped in a standard
tape and reel format.
Package Dimensions
Typical case markings are shown in Figure 30. Package
dimensions for the 8-pin STDFN package are shown in Figure 31.
Tape and reel dimensions are provided in Figure 32.
Care must be taken when attaching this product, whether it is
done manually or in a production solder reflow environment.
Pin 1
Indicator
Skyworks Part #
Figure 30. Typical Case Markings
(Top View)
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DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
0.90 +0.10/–0.15
C
2
0.20 Ref
Seating Plane
A
B
0.45 +0.05/–0.07
Exposed Pad
Detail A
0.02 +0.03/–0.02
Pin 1
Indicator
-B-
2
0.15 C
2X
8X
3
0.85 +0.05/–0.07
Detail C
0.08 C
0.15 C
2X
1.70 +0.10/–0.15
Detail B
2 Places
0.55 + 0.05/–0.04
0.10 C
Top View
Side View
Bottom View
R0.12 Typ
0.25
0.30 ± 0.10
-A-
0.25 +0.05/–0.07
CL
R0.20
Pin 1 Indicator
0.5
5
0.10 M C A B
0.05 M C
Detail C
Scale: 40x
8 Places
Detail B
Detail A
Scale: 20x
Scale: 10x
All measurements are in millimeters.
Dimensioning and tolerancing according to ASME Y14.5M-1994.
Coplanarity applies to the exposed heat sink slug as well as the terminals..
Dimension applies to metalized terminal and is measured between 0.15 mm and 0.30 mm from terminal tip.
S1945a
Figure 33. SKY87201-11 8-Pin STDFN Package Dimensions
4.0 ± 0.1
2.00 ± 0.05
1.5 ± 0.1
3.50 ± 0.05
1.75 ± 0.1
2.3 ± 0.1
0.29 ± 0.06
2.3 ± 0.1
4.0 ± 0.1
Pin 1
Indicator
S3093
Figure 34. SKY87201-11 Tape and Reel Dimensions
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8.1 ± 0.2
0.75 ± 0.1
DATA SHEET • SKY87201-11 LOW-NOISE STEP-DOWN CONVERTER
Ordering Information
Model Name
SKY87201-11 Low-Noise, Step-Down Converter with
Output Auto Discharge
Manufacturing Part Number
SKY87201-11-370LF
Evaluation Board Part Number
SKY87201-11-370LF-EVB
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